Cell biology Ch. 15

You are working in a biotech company that has discovered a small-molecule drug called H5434. H5434 binds to LDL receptors when they are bound to cholesterol. H5434 binding does not alter the conformation of the LDL receptorOs intracellular domain. Interestingly, in vitro experiments demonstrate that addition of H5434 increases the affinity of LDL for cholesterol and prevents cholesterol from dissociating from the LDL receptor even in acidic conditions. Which of the following is a reasonable prediction of what may happen when you add H5434 to cells?

Cytosolic cholesterol levels will decrease relative to normal cells. Normally, cholesterol dissociates from the LDL receptor in the acidic environment of the endosomes and is released into the cytosol. If the drug prevents cholesterol from dissociating from the LDL receptor in acidic conditions, cholesterol may not become released into the cytosol, and thus cytosolic cholesterol levels are likely to decrease relative to those in normal cells. There is no reason to believe that the LDL receptor will remain on the plasma membrane, because the cytosolic region of the receptor is not directly altered by the drug. Vesicle uncoating is also unlikely to be altered, because this occurs after vesicles are pinched off from the membrane.

Your friend works in a biotechnology company and has discovered a drug that blocks the ability of Ran to exchange GDP for GTP. What is the most likely effect of this drug on nuclear transport?

Nuclear transport receptors would be unable to release their cargo in the nucleus. When Ran-GTP binds to the nuclear transport receptor, cargo is released. If Ran could not exchange its GDP for GTP, this would not happen. Ran-GTP is not needed for cargo binding, for nuclear entry, or for interactions with the nuclear pore fibrils during nuclear import.

You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statements must be TRUE?

Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded. ER signal sequences are removed from ER luminal proteins once they are translocated into the cytosol. ER signal sequences are typically at the N-terminus (and not the C-terminus) and bind to SRP. Hydrophobic stop-transfer sequences are found on membrane-inserted proteins and not on soluble proteins. More than one ribosome can bind to an mRNA molecule.

Which of the following statements about a protein in the lumen of the ER is FALSE?

Some of the proteins in the lumen of the ER can end up in the plasma membrane. Plasma membrane proteins come from proteins in the ER membrane, not from the ER lumen.

Figure below shows the organization of a protein that normally resides in the plasma membrane. The boxes labeled 1 and 2 represent membrane-spanning sequences and the arrow represents a site of action of signal peptidase. Given this diagram, which of the following statements must be TRUE?

The C-terminus of this protein is cytoplasmic. The mature version of this protein will span the membrane once, with membrane-spanning segment 2 in the membrane and the C-terminus facing the cytoplasm.

Which of the following statements about vesicular membrane fusion is FALSE?

The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion. Rab proteins are important for docking, but are not involved in the catalysis of membrane fusion.

Cells have oligosaccharides displayed on their cell surface that are important for cell-cell recognition. Your friend discovered a transmembrane glycoprotein, GP1, on a pathogenic yeast cell that is recognized by human immune cells. He decides to purify large amounts of GP1 by expressing it in bacteria. To his purified protein he then adds a branched 14-sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1. Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem?

The oligosaccharide needs to be further modified before it is mature. Oligosaccharides are usually further modified by enzymes in the ER and the Golgi before the glycoprotein is inserted into the plasma membrane. The other choices are untrue, and thus are not good explanations. Oligosaccharides are added to the Asn and not the Ser and are added as a branched 14-sugar oligosaccharide. Disulfide bonding occurs between cysteines of proteins and not in sugars.

After isolating the rough endoplasmic reticulum from the rest of the cytoplasm, you purify the RNAs attached to it. Which of the following proteins do you expect the RNA from the rough endoplasmic reticulum to encode?

all of these answers are correct. The rough ER consists of ER membranes and polyribosomes that are in the process of translating and translocating proteins into the ER membrane and lumen. Thus, all proteins that end up in the lysosome, Golgi apparatus, or plasma membrane or that are secreted will be encoded by the RNAs associated with the rough ER.

Proteins that are fully translated in the cytosol and lack a sorting signal will end up in the

cytosol. Proteins produced in the cytosol that lack sorting signals remain in the cytosol. Proteins produced in the cytosol and destined for the mitochondria or the interior of the nucleus will have a sorting signal to direct the protein to its proper location. Proteins destined for the nuclear membrane are not translated in the cytosol.

Your friend has just joined a lab that studies vesicle budding from the Golgi and has been given a cell line that does not form mature vesicles. He wants to start designing some experiments but was not listening carefully when he was told about the molecular defect of this cell line. He is too embarrassed to ask and comes to you for help. He does recall that this cell line forms coated pits but vesicle budding and the removal of coat proteins do not happen. Which of the following proteins might be lacking in this cell line?

dynamin. Given that coated pits can form but no vesicle budding is seen, dynamin is the most likely answer. Since coated pits are formed, clathrin and adaptin are unlikely to be the answer, because they are involved in the initial shaping of the vesicle into the pit. Rab proteins are involved in the recognition of the transport vesicle with its target membrane and not with vesicle budding.

Signal sequences that direct proteins to the correct compartment are

encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination.Signal sequences are found within the amino acid sequence of proteins. They are sometimes removed when the protein is at the correct destination, but not all are removed. For example, nuclear import signals are not removed once a protein is inside the nucleus. A protein translocator resides in the membrane and helps transport soluble proteins across the membrane, but does not add signal sequences to proteins.

Where are chloroplast proteins translated?

in both the cytosol and the chloroplast. Proteins in the chloroplast are translated in the cytosol and in the chloroplast. The chloroplast proteins that are encoded by the nuclear DNA are translated in the cytosol, and the sorting signals on the protein direct them to the chloroplast. The chloroplast proteins encoded by the chloroplast DNA are translated on ribosomes inside the chloroplast.

In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins?

in the cytosol. Ribosomes are cytoplasmic proteins and thus their protein components are translated in the cytosol.

Proteins that are fully translated in the cytosol do not end up in

transport vesicles. Proteins destined for transport vesicles will be translated on ribosomes associated with the endoplasmic reticulum.